No need for robots.txt files, add contextual robot meta headers.
$ composer require cdaguerre/robots-bundle
dag_robots:
rules:
- { route: 'homepage', tags: ['noindex', 'nofollow'], hosts: ['www.example.com'] }
- { route: 'product_show', tags: ['noindex', 'nofollow'], hosts: ['www.example1.com', 'www.example2.com'] }
- { route: 'category_show', tags: ['noindex', 'nofollow'] }
- Add tests